The invention relates to a method of automatically checking the form of workpieces produced in large numbers, and to an apparatus for carrying out the method. Its object is more particularly to compare workpieces in respect of their form with a specimen, the said workpieces being continuously fed at a high rate. The comparison permits a simple decision as to whether the workpieces are acceptable or unacceptable, i.e., as to whether their shape does or does not conform well with that of the specimen.
Usually, this is done by gauging the critical dimensions or by visually comparing the individual workpieces with a specimen. The visual comparison takes place without special aids or by means of a measuring microscope, a projection apparatus or like optical enlarging means, depending upon the size of the workpiece and the required tolerances. In the case of workpieces of complicated shape, the application of such visual checking methods is very time-consuming and labour-intensive.
Checking methods in which holography is employed have been developed and are being used for special applications. These holographic methods are carried out with devices which were originally developed for character recognition in accordance with the proposal of A. Vander Lugt in the year 1964 (IEEE Trans. Inf. Theory IT-10,139).
This method is based upon the following principles: an object is illuminated by a beam of coherent light. The object beam scattered by the object contains all the information regarding the shape of the object. A reference beam emanating, for example, from a point source is superimposed upon the object beam. The object beam and the reference beam, or the interference pattern formed, is fixed on a photographic plate, i.e., the hologram, which thus contains all the information regarding the object. If the hologram is again illuminated only by the object beam, i.e., the light beam scattered by the object, there is set up beyond the hologram a light beam which is identical with the reference beam used for the recording, that is to say, the reference beam is reconstructed. If the latter has originated from a point source, there is then again obtained a reconstructed point source. If, instead of the object, an article to be tested identical with the object is introduced into the beam emanating from the light source, a point source is again reconstructed. Should the article to be tested differ from the object in its geometry, a reconstructed point source is not obtained, but a more or less large spot. The size of this spot, or the reduction of the intensity at the centre, is hence a measure of the geometrical difference between the tested article and the object.
The further development of this testing method is based upon the principles of Fourier optics. It is known that if a first lens is disposed, at a distance equal to its focal length, behind an object illuminated by a plane light beam, there is obtained in the rear focal plane a distribution of the light amplitude which corresponds to the spatial two-dimensional Fourier transform of the object. If a complex filter is then disposed in the said plane and a second lens forms on a plane an image of the beam issuing from the said filter, there is here obtained the Fourier transform of the filtered Fourier transform of the object. This transform thus corresponds to the filtered image of the object, because two successive Fourier transformations give the original function with the exception of the sign of the argument and any existing scale factor.
It will now be assumed that the complex filter is produced as follows. The object is again illuminated by a plane coherent light beam. The first lens is situated beyond the object. In its rear focal plane, where the image of the Fourier transform of the object is formed, there is superimposed upon the said transform an obliquely incident plane reference beam coherent with the light beam. The interference pattern produced is recorded on a photographic plate. The developed photographic plate is called the holographic filter of the object.
If the object is replaced by an article to be tested, which is illuminated with the plane light beam, whose Fourier transform is reproduced by the first lens in its rear focal plane by the first lens, the holographic filter of the object is introduced into the said plane and the beam issuing therefrom is transformed by the second lens, there is obtained in the rear focal plane of the latter, on the one hand, the cross-correlation integral of the test piece function with the conjugate-complex object function, and on the other hand at another location on the rear focal plane the convolution integral of the test piece function with the object function. Only the first is of particular interest to use here. If the test piece function is contained in the object function, or if the two are identical, the cross-correlation integral becomes integral with an auto-correlation integral, which is generally distinguished by a sharp, central, intensive light spot.
It is thus possible to ascertain with a light-sensitive element whether a tested article is or is not identical with the object.
Detailed illustrations of the principles and examples of application of this technique are to be found, for example, in the book "Optical and Acoustical Holography," issued by Ezio Camatini, Plenum Press, New York-London, 1972.
A whole number of difficulties arise in the practical application of this method.
The dynamic range of the photographic plate does not permit of faithfully recording the interference pattern of the whole spectrum. In practice, therefore, the best possible compromise must be brought by experiments. It is often even advantageous for the very high space frequencies not to be recorded, because they only contain information regarding the surface nature of the object. Often, the interference pattern of the spectrum is also calculated if the geometry of the object can be simply mathematically described. The holographic filter is then synthesised.
A further difficulty resides in the sensitivity of the installation to errors in the scale and the angular position of the tested article. The first error, which must generally be recognized for monitoring and comparison purposes, can be balanced out by variable focal widths of the lenses in special cases where it is to be disregarded. The second error, which has nothing to do with the shape of the tested article, but is generally due to the difficulty in handling and accurately positioning it, is offset by rotating either the tested article or the holographic filter about the optical axis. However, this is not readily practicable because the rotation must take place in a plane and without vibration. In addition, it requires a time which is often not available in the monitoring of articles produced in large numbers. One object of this invention is to indicate a means by which this difficulty can be readily avoided and which renders possible measurements in rapid succession.
A further family of difficulties resides in the inaccuracy in the positioning of the various elements of the apparatus.
Above all, the position of the holographic filter is essential for obtaining a sufficiently informative signal. The requirements, i.e., the positioning tolerances, are in the order of magnitude of the wavelength of the light employed. It is thus clear that a constant rotation of the filter can be effected only at very great cost.
The position of the article to be tested in the direction of the optical axis is not very critical because, as is known, the installation is rendered translationally constant by the use of the transformation lenses. However, the deviation from the ideal position should be much less than 2l.sup.2 /.lambda., where l is a small characteristic length of the article which is to be monitored, and .lambda. the light wavelength. Normally, tolerances of a few tenths of a millimeter or more are obtained.
Displacement of the tested article perpendicularly to the optical axis results in a displacement of the location of the correlation function on the plane of emergence of the image. The height of the function at the centre is not influenced thereby. Since this height is the magnitude of interest to us, which renders possible information regarding differences in shape of the tested article with respect to the object, it is consequently obvious to fix the location of the correlation spot (abbreviation for the centre of the correlation function) and then to measure the height thereof. This is done, but involves very costly electronic equipment. In addition, there do not at present exist any light-sensitive surfaces which give a like evaluation of a like signal over the whole area. Consequently measuring errors arise, depending upon the location of the correlation spot. In order to eliminate this possibility of error, there is therefore employed a light-sensitive element of very small extent, or a diaphragm having a very small aperture is disposed in front of the element. Since the correlation function can very rapidly decrease, starting from the centre, and as mentioned only the absolute height at the centre is of interest, the correlation spot must be very accurately adjusted to the quasi-point form, light-sensitive element (detector) (or vice-versa). A diaphragm aperture of, for example, 0.01 mm or a light-sensitive element of corresponding size involves a positioning tolerance of the correlation spot of the order of magnitude of 1 .mu.m. With a 1:1 image formation, therefore, the tested article must be positioned with this tolerance, with the detector fixed in position, which is not practicable with mass-produced articles which have to be tested at a high rate.
A further difficulty resides in that certain workpieces can only be pre-sorted the right way round at high cost. Stamped sheet-metal plates of round form having irregularly distributed apertures are one example. It is desirable to be able to carry out a monitoring with such plates in any desired angular position and without any particular plate face (upper or lower) always having to be in the same position.
It is consequently an object of the present invention to indicate a method and an apparatus for the application thereof, by which the requirements as to the accuracy of the positioning of the article to be tested and as to its orientation can be met in a practical manner and which permit comparison in rapid sequence.